1. Field of the Invention
This invention relates to an improvement in grain dryers of the concurrent-countercurrent flow type.
2. Summary of the Prior Art
It is frequently necessary to dry grain before it is stored. Otherwise, the moisture content of the grain may cause discolouration and spoilage during storage.
This need to dry grain prior to storage has long been recognized and many systems have been developed over the years for this purpose. In many of these prior systems, grain is heated quickly to a maximum temperature and then quickly cooled by exposure to air. One of the most difficult problems associated with this type of system has been that the quick temperature changes have tended to result in stress cracking and shattering of the grain. This, of course, greatly lowers the value of the grain such that it may not even be considered acceptable to many grain elevators and processors.
Many systems have been developed over the years which are intended to heat and dry the grain uniformly while at the same time avoiding the problems of stress cracking. One such system is the cross-flow column type grain dryer in which air is transversely forced through the downwardly moving grain in an attempt to evaporate moisture. However, with this type of dryer, great difficulty is encountered in trying to provide a uniform air flow and heating path and generally the grain has been dried very unevenly by this type of system.
Counter flow drying systems have also been widely used in which drying air is forced through the grain in a direction opposite to the direction of grain travel. Of course, with this system the dried grain at the bottom of the bin is exposed to the hottest air and the wet grain at the top is exposed to cooler air. While this is the most efficient drying method now available, it does have the rather serious problem that some of the grain is overdried, resulting in cracking of the kernels and lowering of quality.
A more recent development which is proving to be quite successful is the concurrent-countercurrent flow grain dryer in which hot drying air travels downwardly in the same direction as the flowing grain and a countercurrent flow of cooling air travels in a direction opposite to the direction of the grain travel. With this system, air exhaust means are provided intermediate the hot air inlet and cooling air inlet. It will be seen that with this system, the hottest air is used at the point where it is of greatest value, i.e. where the grain is wettest and coolest, and as the two travel together, the air heats and dries the grain while the grain gradually cools the air. The counter flow of cooling air serves to further cool and temper the grain before it reaches the grain outlet at the bottom of the bin.
One of the earliest concurrent-countercurrent grain dryers is described in Oholm U.S. Pat. No. 2,706,343. However, the Oholm system has the disadvantage of presenting a V-shaped grain surface area exposed to the entering hot air. This V-shaped surface area of the grain bed results from the grain falling free from a feed spout into a pile having sloping conical sides. Thus, the length of time that individual particles are exposed directly to the hot air will vary according to the particle position on the pile, with those embedded near the peak of the pile being exposed for a shorter period than those embedded on the side slopes.
In prior practice in order to arrive at some practical acceptable average moisture level for most of the grain flowing through the bed, it has been necessary to over dry at least some of the grain.
An effort to overcome such a problem is described in Anderson, Canadian Pat. No. 940,295 in which a means is provided by which grain to be dried is periodically introduced to the drying bed and evenly spread over the entire area of the hot air inflowing end of the bed, as in successive flat layers, without exposing the wet grain being added to the heated dry air until the wet grain being added has been deposited on the bed.
The Anderson system is a reasonably efficient drying system and can produce a good quality dried grain but suffers from being very expensive to construct and also provides operational difficulties because of the structural complexity of the movable sweeps used for evenly distributing the incoming wet grain across the drying bed.
A substantial improvement over the Anderson system is described in Westelaken, U.S. application Ser. No. 770,048 filed Feb. 18, 1977, now U.S. Pat. No. 4,086,708. It describes an improved wet grain-hot air contacting arrangement and includes a wet grain bin having a horizontal floor assembly with a plurality of uniformly spaced openings, with a tube member extending downwardly beneath each such opening. These tubes serve to deliver wet grain in response to gravity from the bin into a drying chamber. A hot air inlet duct is provided adjacent the tube members to deliver hot air into the space between the tube members and downwardly through a bed of grain in the drying chamber. The bottom of the drying chamber has metering grain outlets as well as cooling air inlet ducts, while air exhaust ducts are provided intermediate the hot and cooling air inlets. The novel floor assembly permits the wet grain to be cyclically delivered into the drying chamber with a pulsating action, which causes a lateral flow resulting in uniform layers of wet grain being deposited in the drying chamber, where it comes into contact with the hot air. That system has shown very substantial improvements both in the efficiency of moisture removal and the quality of the dried grain.